ARM: dts: imx51-zii-scu2-mezz: Disable the internal watchdog

imx51-zii-scu2-mezz has an external watchdog in the environment
microcontroller, so disable the internal one.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>